Existing Soundscapes and the Impact of Noise on the Welfare of Farmed Salmonids: A Review

ABSTRACT Understanding how farmed salmonids sense and respond to their environment is key to securing animal welfare and efficient production. However, hearing has rarely received attention, despite the omnipresence of underwater sound and its potential effects. We describe how and what salmonids hear, and explore how hearing may differ between wild and farmed conspecifics. We then conduct two systematic reviews, to: (1) understand the existing soundscapes experienced by farmed salmonids; and (2) identify potential short‐ and long‐term impacts of sound on salmonid behaviour and physiology. Sound can be measured via pressure changes or particle motion; pressure levels are commonly reported, whilst particle motion is rarely measured due to logistical challenges. Salmonids have an open, physostomous swim bladder, relying primarily on their lateral line to sense sound via particle motion. Because of this, they are most sensitive to low frequencies, with startle responses most apparent between 7 and 400 Hz. Few studies have tested for effects of sound on salmonid physiology, although there is evidence that some sound exposure regimes lead to chronic stress, whilst very loud sounds (e.g., cumulative sound exposure level > 216 dB re 1 μPa 2 a) can cause fatal injuries. Quantifying salmonid aquaculture soundscapes, their potential welfare impacts, and strategies to mitigate these impacts should form a new frontier in salmonid aquaculture research. We make a series of recommendations, specifically: (1) conduct targeted research to fill knowledge gaps; (2) develop a standardised protocol to measure and report sound in salmonid aquaculture; and (3) apply mitigation strategies to reduce impacts of noise.